1. Field of the Invention
The present invention relates to a shock absorber for absorbing a shock.
2. Description of the Related Art
JP-A-63-277859 discloses a starter equipped with a shock absorber for interrupting transmission of an excessive torque. In such a shock absorber for a starter, a rotary disk is pressed against a stationary disk to generate a frictional force so that rotation of the rotary disk can be controlled. Accordingly, when a shock that is more than a certain normal torque is applied to the rotary disk, the rotary disk slips or rotates, thereby absorbing the shock.
Such a rotary disk has a plurality of dimples and small banks formed around the dimples at a height of about tens of microns, providing a press-formed frictional surface. An amount of grease is filled in the spaces (dimples) between the stationary disk and the rotary disk with the banks being in contact with the surface of the stationary disk to prevent seizing.
Because an array direction of the dimples is the same as the direction of the rolling of the raw material of the rotary disk, the height of the banks are not formed even. That is, the bank of the sides in parallel with the direction of rolling is lower than the bank of the sides that is perpendicular to the direction of rolling. Only two sides of the rectangular dimples can touch the stationary disk. Therefore, the two side-banks may be worn away more easily than the four side-banks. This lowers the lifetime of the shock absorber.
If another two-side banks start touching the stationary disk after the first two side-banks initially touched the stationary disk have worn away, the friction factor changes too abruptly to control the transmission torque.
As the rotary disk has rotated for a long time, the spaces for the grease between the head of the dimple and the bottom thereof become narrower. Accordingly, a sufficient amount of grease cannot be supplied to the sliding surfaces of the disks. This causes the seizing and shortens the lifetime.